Question 1

Which of the following statements best describes the law of conservation of electric charge?

Accepted Answer

Electric charge is neither created nor destroyed; it can only be transferred from one object to another.

Question 2

A plastic rod is rubbed with wool, and then brought near small pieces of paper, which are attracted to the rod. What is the best explanation for this phenomenon?

Accepted Answer

The rod becomes negatively charged and induces a separation of charge in the paper, resulting in attraction.

Question 3

Which of the following materials is a good conductor of electricity?

Accepted Answer

Copper

Question 4

A glass rod is rubbed with silk. What type of charge does the glass rod acquire, and why?

Accepted Answer

Positive, because it loses electrons to the silk.

Question 5

Which equation correctly expresses Coulomb’s Law for the magnitude of the force between two point charges $$q_1$$ and $$q_2$$ separated by a distance $$r$$?

Accepted Answer

$$F = K \frac{q_1$ q_2$}{r^2$}$$

Question 6

Two point charges, $$+2\ \mu C$$ and $$-2\ \mu C$$, are placed 10 cm apart. What is the direction of the force on the positive charge?

Accepted Answer

Toward the negative charge

Question 7

A neutral atom becomes a positive ion when:

Accepted Answer

It loses an electron.

Question 8

Which of the following best describes the electric field at a point in space?

Accepted Answer

It is the force per unit charge that a test charge would experience at that point.

Question 9

A parallel-plate capacitor creates a uniform electric field between its plates. Which equation gives the magnitude of this field?

Accepted Answer

$$E = \frac{Q}{A \$$epsilon_0$$}$$

Question 10

A small glass bead of mass $$15\ \mathrm{mg}$$ and charge $$+10\ \mathrm{nC} is $$placed 1.0 cm below a plastic sphere with charge $$-10\ \mathrm{nC}. $$What is the ratio of the electric force to the gravitational force on the bead? (Use $$K = 9.0 	imes 10^9$\ \mathrm{N\cdot m^2$/C^2$$}$$ and g = 9.8$$\ \mathrm{$$m/s^2$$}$)

Accepted Answer

About 60 (electric force is much larger)

Question 11

Which of the following statements about electric field lines is FALSE?

Accepted Answer

Field lines can cross each other.

Question 12

A negatively charged rod is brought near, but does not touch, a neutral metal sphere. What happens to the charges in the sphere?

Accepted Answer

Negative charges in the sphere are repelled from the rod, leaving the near side positively charged.

Question 13

Which of the following is TRUE about the electric field inside a conductor in electrostatic equilibrium?

Accepted Answer

The electric field is zero everywhere inside the conductor.

Question 14

A DNA fragment with a negative charge is placed in a gel electrophoresis apparatus with a uniform electric field pointing to the right. In which direction will the DNA fragment move?

Accepted Answer

To the left, opposite the field

Question 15

Which of the following best describes the force on a dipole placed in a uniform electric field?

Accepted Answer

There is no net force, but there is a net torque.

Question 16

A housefly with a mass of $$12\ \mathrm{mg}$$ and a charge of $$+73\ \mathrm{pC} is to be $$levitated by an electric field. What is the required field strength? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$E = \frac{mg}{q} = \frac{12 	imes 10$$^{-6}$$\ \mathrm{kg} 	imes 9.8\ \mathrm{m/s^2$}}{73 	imes 10$$^{-12}$$\ \mathrm{C}} \approx 1.6 	imes 10^6$\ \mathrm{N/C}$$

Question 17

Which of the following is a real-world application of electrostatics discussed in the chapter?

Accepted Answer

Photocopy machines use electric fields to attract toner particles to charged areas of a drum.

Question 18

If two point charges are separated by a distance $$r$$ and the force between them is $$F$$, what will be the new force if the distance is doubled?

Accepted Answer

The force will be one-fourth as large.

Question 19

Which of the following best describes the direction of the electric field produced by a positive point charge?

Accepted Answer

It points radially outward away from the charge.

Chapter 20: Electric Fields and Forces – Study Notes

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